Manually operated, window shades or coverings presently in wide scale commercial use can be classified as either cord type or cordless.
Cord type arrangements are universally used today to operate Venetian blinds, cellular shades, pleated shades, thermally insulated shades, Roman and balloon type shades. In all of these shade arrangements, no less than two cords (or a single looped cord) hang from the side of the shade and extend into a head rail which carries some form of pulley arrangement. The pulley arrangement attaches to a set (typically two or three) of draw cords or strings which extends through openings in the blind or shade and attaches to the bottom rail. Pulling the side cords shorten the draw strings so that the bottom rail draws against the head rail and in the process folds or pleats the shade or blind. Lateral movement of the side cords locks and unlocks the pulley so that the weight of the shade or blind will cause the shade or blind to lower. If the lower rail is uneven, pulling one of the side cords shortens one of the draw strings to bring the bottom rail even. (Venetian blinds typically have a wand on the opposite side of the head rail and additional draw strings so that rotation of the wand causes the individual slats to open or close.) An inherent advantage in cord type shades is that the length of the shade can be accurately sized to the length of the window opening. This prevents the shade from bunching or gathering in a folded condition when the shade is fully drawn to span the window openings.
There are numerous lock and clutch mechanisms used with cord shades, and many clutch mechanisms utilize springs to assist in raising or lowering the shades. Examples of such systems may be seen in U.S. Pat. No. 4,623,012, issued Nov. 18, 1996 to Rude et al and U.S. Pat. No. 5,228,491 issued Jul. 20, 1973 to Rude et al. In cord operated mechanisms for window coverings, the lift or ladder cords and, in some applications, the draw cords also are either wound about a stationary capstan or a laterally moving capstan. The laterally moving capstan offers the advantage of wrapping the cords in a single layer. There are numerous mechanisms for winding the cords about a stationary capstan and numerous mechanisms for laterally moving the capstan.
Even though mass production techniques have reduced the price, the cord type arrangement is relatively expensive and typically requires installation by trained installers. The cords hanging down from sides of the shades are aesthetically unpleasing, especially so in window installations where several windows are adjacent one another. The most serious drawback, however, is the safety hazard caused by the dangling cords. The industry has recognized this problem as evidenced by U.S. Pat. No. 5,133,399 to Hiller et al, U.S. Pat. No. 5,485,875 to Genova and the earlier U.S. Pat. No. 3,485,285 to Anderle. Recently, voluntary safety standards for eliminating or reducing the potential strangulation hazzards posed by looped window treatment cords have recently been adopted by The Window Covering Manufacturers Association and approved by the American National Standards Institute.
The old fashioned, conventional roller shade bar window shade is universally accepted as the cordless window shade of choice It is low cost and installed by the homeowner or end user and does away with the problems of a cord. Because the shade must roll onto itself over the roller shade bar, it can not be used for Venetian blinds, cellular shades, pleated shades thermally insulated shades, Roman and balloon type shades. Furthermore, many of these applications have window covering weights which exceed the spring tension generated by conventional roller shade bar springs. In fact, because of this limitation, cord drawn shades with spring assisted clutches have been developed.
Nevertheless, the prior art has attempted to develop cordless applications for such shades. The cordless prior art can be viewed as falling within one of three different design approaches or classifications.
In the first approach, spring or motor driven pulley arrangements are arranged at the sides of the shades or outside the head rail of the shade. The shades are basically raised and lowered by rollers in tracks mounted on the side of the window not entirely dissimilar to how a garage door opens and closes. See for example U.S. Pat. No. 5,141,041 to Katz et. al; U.S. Pat. No. 4,880,045 to Stahler and U.S. Pat. No. 4,862,941 to Colson. For side guides see also, U.S. Pat. No. 4,625,786 to Carter et. al; U.S. Pat. No. 4,398,585 to Marlow; U.S. Pat. No. 2,325,992 to Wirthman and 948,239 to McManus. In U.S. Pat. No. 2,324,536 to Pratt a spring on the support rod provides tension for raising and lowering the Venetian blind by moving the bottom rail through tapes outboard of the slats contained in enclosed side columns. In this approach, the side rails simply make such devices cost prohibitive and unwieldily except for special applications.
The second approach can be summarized as using a spring to wrap the shades lift cords about a sheave mounted in the head rail. One variation of this design approach is disclosed in U.S. Pat. No. 5,482,100 to Kuhar. The concept is perhaps best illustrated in U.S. Pat. No. 3,487,875 to Shukat et al which discloses a roller bar drawing lift cords about sheaves. Wrapping lift cords about themselves will not maintain the bottom rail parallel with the head rail. Earlier designs are disclosed in U.S. Pat. No. 2,037,393 to Roberts; U.S. Pat. No. 2,266,160 to Burns; U.S. Pat. No. 2,276,716 to Cardona; and U.S. Pat. No. 2,824,608 to Etten. The earlier references disclose Venetian blinds having a conventional ratchet pawl spring mechanism in which a cord or a thin tape within the ladder tapes straddling the slats are would onto a sheave attached to the roller bar for raising and lowering the blind. In Letten the cord is outboard. In Roberts the ladder tape itself is wound through pulleys onto the rod and the ladder tape has notches for engaging a mechanism for changing the vertical orientation of the slats held by the ladder tape. The weight of the slats of Venetian blinds, even considering today's light weight mini-blind materials, make the choice of a conventional, ratchet/pulley spring mechanism unacceptable. To obtain a sufficiently strong spring which can be inserted inside a conventional roller shade bar, the diameter of the bar has to be significantly increased. Increasing the diameter of the roller bar means the head rail depth has to be increased accordingly and the resulting aesthetics would not be acceptable in today's home market. Apart from aesthetic considerations, the prior art devices could not maintain the bottom rail consistently parallel with the roller shade bar during repeated operation of the blind. In today's market a variation in alignment of the bottom rail as little as 1/4 inch in a shade having a width as short as 36 inches will not be tolerated. While the prior art devices could probably achieve this alignment, initially, inevitably misalignment caused by repeated shade use, will occur. Further, should the bottom rail move out of alignment, there is no way to adjust the blind to bring the bottom rail into alignment.
In the last category of spring drawn shades are those in which the shade bar transversely moves within the head rail to cause the lift cords to wrap and unwrap about a capstan as the shade is lowered and raised. U.S. Pat. No. 5,133,399 to Hiller et al shows such an arrangement in which the lift cords are spirally wound onto cone shaped capstans with spiral grooves formed therein. As the lift cords are wrapped into the grooves, the shade bar, which is mounted onto a platform within the head rail, laterally moves with the platform within the head rail. Significantly, the size of the cone shaped capstan and the tension of the spring are selected to counterbalance one another so that the spring tension maintains the shade at any drawn position of the shade. While it is doubtful that the calibration between spring and capstan can be maintained, the size of the cone shaped capstan coupled with the mounting arrangement will result in an excessively large head rail deemed unacceptable in today's market.